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Zoning is often viewed as a friend of conservation. But could some zoning actually be detrimental to conservation goals?

It may seem almost heresy for environmentalists to argue for more development in certain places. But that’s exactly what I will argue in this blog. Let me explain.

On a recent trip to the San Francisco Bay Area, I saw a worrisome sight I often see in America cities. In North Berkeley, you can be just a few blocks from a Bay Area Rapid Transit station, which offers an easy 30-minute commute into downtown San Francisco.

And yet the neighborhood is single-family, detached homes that house relatively few people. Zoning has made it so, by prohibiting certain kinds of structures (apartment buildings, for instance) that could house way more people.

This is sometimes called exclusionary zoning, when the primary purpose is to restrict further development or change in a neighborhood. It’s a change from (arguably) the original purpose of zoning and building codes, which was to protect public health and safety (by preventing a factory with smokestacks being built next to residential housing, for instance). It’s also massively common in the United States—one study classified 80% of U.S. urban land as having minimum lot sizes that prevent more dense development. On some parcels, that may be quite appropriate, for environmental, health or safety reasons. But it is striking that four out of five landowners are prevented from developing more densely, even if they wanted to.

Environmentalists have to tread carefully when discussing zoning codes. We want the government to be able to use zoning codes to protect public health, ensure access to public parks and protect at least a few parcels of open space in a metro area. We want governments to be able to plan to make cities more walkable, greener, healthier places. So we respect and support strong zoning codes. Yet the massive exclusionary zoning in many cities has restricted housing supply near cities, pushing up prices there and contributing to urban sprawl, and the spread of new low-density neighborhoods in the fringes of urban areas.

One study I worked on in the Bay Area looked at this process. Parks in the Bay Area, while extensive, didn’t restrict the housing supply much, since land protection was overwhelmingly on sites that were either too steep or too wet to support dense development. The main limitation on housing supply was simply the large amount of area restricted by zoning to single family homes.

The Bay Area is actually denser already than many U.S. metro areas, and regional groups like the Greenbelt Alliance are actively trying to encourage infill development, increasing housing density within the urban footprint of a city rather than expanding the urbanized areas. The Association of Bay Area Governments has mapped places for infill development, and outlines changes to zoning and tax codes that could make infill development more possible. The Metropolitan Transportation Commission has promoted transit-oriented development, to support this infill development. Despite all these efforts, I was struck during conversations in the Bay Area by how controversial infill development can be. Change is hard. It is hard for people to accept changes in neighborhoods they have come to know and perhaps love.

I have come to believe that in the proper places, environmentalists need to argue for allowing more development. As part of a well-planned city, allowing more density near urban centers would prevent sprawl and economically revitalize urban centers.

It will limit suburban sprawl, and help protect biodiversity in remnant habitat patches on the fringes of the city. Development alone won’t, of course, ensure that housing prices are affordable, and cities will also need to have programs to ensure some affordable housing for lower income residents. But allowing more housing units on previously low-density parcels will help reduce housing prices somewhat city-wide.

The alternative to infill development is clear: U.S. cities will continue to sprawl out. While much has been written about the preference of millennials for urban life, the overwhelmingly majority of new houses in U.S. cities are still built on the urban fringes. One of the greenest things U.S. cities could do is get a bit denser. This push for infill development and walkable development is not a new thing for urban planners and new urbanists. But for the conservation movement it is still a new, sometimes hard step, to openly call for more development in certain key, appropriate places.

I saw this in my own city of Washington, D.C., where there was an effort to develop a light rail line (the Purple Line) on a previously cleared old railroad right-of-way, bought by the city decades before explicitly for this purpose.

This effort has been delayed and stymied by landowners along the route worried about the loss of a few hundred trees along the rail line, and the change that will come to their neighborhood as more people can access it. Some local environmental groups have fought the Purple Line, to protect those few hundred trees, but most have fought to allow the rail line to happen. Infill and denser development have to happen for the greater environmental good, and it is a sign of maturity for the conservation movement to be willing to say so.